The present invention relates to passive geophysical exploration and more particularly to determination of subsurface discontinuities associated with variance in low frequency electromagnetic fields emitted from, or reflected by, the earth.
It has long been known that there are substantial electromagnetic fields associated with the earth. The origin of these electromagnetic fields is unclear: one theory holds that low-frequency electromagnetic fields are emitted from beneath the surface of the earth and radiate outward such that they can be measured by low frequency method at the surface. Others postulate that currents are generated by oxidation-reduction reactions taking place where water and hydrocarbons are present, and that the electromagnetic radiation is caused by interaction of these steady and unsteady currents with the earth""s magnetic field. Still others postulate that the radiation is reflected from outside the earth""s atmosphere.
According to well-documented scientific theory, however, discontinuities in the subterranean structure of the earth crust cause reflection of electromagnetic radiation at interfaces between electrically different materials. Additionally, the distance a transverse electromagnetic wave travels in a material before being substantially absorbed, is a known function or the frequency of the wave (so-called xe2x80x9cskin-depthxe2x80x9d expressions). Thus it has been hypothesized that prospecting for hydrocarbons such as oil, gas and coal, as well as precious metals, could be achieved by mapping the strength of electromagnetic waves at various frequencies which emanate from the earth providing a passive method of subterranean exploration and prospecting techniques.
It is well known that there are many active methods for determining geologic subterranean surfaces. It is standard in the oil and gas industry to set off pre-programmed charges, sending a mechanical wave through the area to be mapped, and then receiving these waves with sensitive seismic meters to locate and identify subterranean geologic formations. This methodology employs the concept that discontinuities in subterranean structure reflect mechanical waves, and that different wave frequencies propagate differently in the earth, similar to the behavior of electromagnetic waves described earlier. Grounded in scientific theories of mechanical wave propagation, a skilled artisan can process the received mechanical wave patterns, into a xe2x80x9creadablexe2x80x9d form and then use that information to determine with some precision the location of specific geologic structures which have been shown empirically to relate to hydrocarbons such as oil, gas, coal or other materials. Thus, these deposits can be pinpointed with some accuracy such that exploration drilling and the like can be optimized.
Many different methods for picking up and determining low frequency electromagnetic waves emanating from the earth have been proposed. By utilizing an antenna to pick up these naturally occurring frequencies emanating from the earth surface one can typically filter, amplify, modify and otherwise process these signals to turn them into a readable form.
Several prior art methods of detecting hydrocarbons from the earth""s radiated low frequency signals have been proposed. Typically the received signal is amplified, filtered and detected using analog techniques. U.S. Pat. No. 5,148,110 issued to Helms detects a time varying signal emanated from the earth""s surface. U.S. Pat. No. 4,686,475 to Cober et al. detects the vertical electric field components of telluric currents using a special antenna and a tuneable filter, with field detection performed in an audio manner using a simple listening device. As the frequencies monitored moves across a range of frequencies, the pitch heard by the operator varies. This method, however, is extremely subjective and suffers from unreliability and inconsistency.
It well known that there is an extremely low signal-to-noise ratio associated with these low frequency signals, and this high level of noise typically causes interference in detecting those signals that are determinative of geologic formations. Various low pass and high pass filtering techniques have been used or employed after initial amplification of the signal. This amplification and filtering is know as conditioning and pre-conditioning of the signal and is generally considered to be the most common technique for identifying electromagnetic signals emanating from the earth.
One of the challenges in the prior art, then, has been to find a proper way of conditioning the received signal so that the xe2x80x9ctruexe2x80x9d information can be determined in a consistently reliable and repeatable manner. One problem commonly experienced in the prior art is that simple application is not typically sufficient to allow filters to operate effectively. U.S. Pat. No. 3,087,111 issued to Lehan at al describes a system whereby signals are amplified and then added to the oscillator frequency. Additionally, the background scientific basis for the method in U.S. Pat. No. 3,087,111 assumes that the skin-depth relation for the earth is       δ    =                  2        μσω              ,
(in MKS Units) an approximation that is valid only if the earth is a very, very good conductor (i.e. the conductivity "sgr" is large). However, it is well accepted that the earth is not a good conductor.
U.S. Pat. Nos. 3,1797,704 to Simon, et al. and 4,198,596 to Waeselynck, et al. describes a system that adds an oscillator frequency to the detected signal and then low pass filters. For the most part the oscillator controls the center pass frequency of the filter being used. Similarly, U.S. Pat. No. 6,087,833 issued to Jackson describes a geophysical prospecting technique that uses a tuned signal to sweep a variety of sensed frequencies.
The major problem in any of these prior art techniques is analyzing the data. Because of the low frequency and energy level of these signals, separating this data from background noise is particularly difficult. Each of these prior art devices and techniques is susceptible to noise problems emanating from spurious signals from external noise sources such as distant lightning strikes, airplanes passing overhead, cellular and wireless phones, and the like. Moreover, each of these devices and techniques make use of analog signal processing techniques such as modulation/demodulation. These analog techniques are typically clumsy and require relatively high-energy electromechanical devices such as strip-chart recorders. These electromechanical devices are typically bulky and consume battery power quickly, thus making them unsuitable for use in the field for prolonged periods of time. It is therefore desirable to create a geophysical exploration device that overcomes the various shortcomings of the prior art.
In accordance with various aspects of the present invention there has now been discovered a method and apparatus for analyzing electromagnetic radiation which emanates from the earth by use of analog to digital conversion and matched anti-aliasing filtering so as to compile data which is stored in a buffer and then Fourier transformed to render a readable signal such as on a display. Various embodiments of the invention account for a signal with a spectrum between about 0 Hz and 5 kHz that can be produced without using a modulator or demodulator or a comparator. In accordance with the invention electromagnetic waves emanating from the earth are received by an antenna digitally sampled and stored. A Discrete Fourier transform (such as a fast Fourier transform (FFT)) converts time domain information into frequency domain information for display and further analysis.
In accordance with the broad aspect of the invention, low frequency electromagnetic energy emanating from the earth is received, filtered, digitized, and transformed into a readable signal. The device of the instant invention employs an antenna to receive an electromagnetic signal which is passed through a low pass filter and buffer to a tuned analog to digital (A/D) converter, which allows the received signal information to be transformed or otherwise processed digitally the converted digitized signal may be stored in a buffer such as a hard drive, and may be displayed on any display means such as a CRT, flat panel display, printer or the like.
In accordance with the method of the instant invention, electromagnetic radiation is received from the earth, is filtered to remove high-frequency components, converted to a digital equivalent, and processed to produce an analyzable output.